Wells are generally drilled into subsurface rocks to access fluids, such as hydrocarbons, stored in subterranean formations. The formations penetrated by a well can be evaluated for various purposes, including for identifying hydrocarbon reservoirs within the formations. During drilling operations, one or more drilling tools in a drill string may be used to test or sample the formations. Following removal of the drill string, a wireline tool may also be run into the well to test or sample the formations. These drilling tools and wireline tools, as well as other wellbore tools conveyed on coiled tubing, drill pipe, casing or other means of conveyance, are also referred to herein as “downhole tools.” Certain downhole tools may include two or more integrated collar assemblies, each for performing a separate function, and a downhole tool may be employed alone or in combination with other downhole tools in a downhole tool string.
Formation evaluation may involve drawing fluid from the formation into a downhole tool. In some instances, the fluid drawn from the formation is retained within the downhole tool for later testing outside of the well. In other instances, downhole fluid analysis may be used to test the fluid while it remains in the well. Such analysis can be used to provide information on certain fluid properties in real time without the delay associated with returning fluid samples to the surface.
Asphaltenes are generally the heaviest fraction and the most polar component in a petroleum mixture. Asphaltenes are insoluble in n-alkanes and soluble in toluene. They can be aggregated as solid particles under certain pressure and temperature conditions in some crude oils, or by injecting a solvent. During production, the solubility of asphaltenes in crude oil decreases with pressure as the fluid travels through the reservoir and the wellbore. Asphaltene deposition can sometimes negatively impact production, such as by plugging pipelines and impairing the flow of hydrocarbons through formations and into wells.